1. Field of the Invention
This invention relates to marine seismic streamer cables such as might be found in class 367/15.
2. Discussion of the Prior Art
Conventional marine seismic streamer cables usually consist of a hose-like tubular plastic jacket of circular cross-section, about 6.5 cm in diameter, that contains a plurality of hydrophones mounted therein at intervals of perhaps, one meter. The hydrophones are electrically interconnected into arrays of one or several hydrophones per array. The streamer cable is usually divided into sections on the order of 100 meters long. Each section may include 10 or more hydrophone arrays. A number of individual sections, perhaps as many as 50, are interconnected to form the complete streamer cable.
Seismic signals from the respective hydrophone arrays are transmitted to a multi-channel signal utilization device aboard the ship that tows the streamer cable through a body of water. The seismic signals from each array may be transmitted as analog signals through individual electrical conductors or they may be transmitted as digital signals through a telemetric system. Regardless of the type of signal transmission, the combined weight of the electrical conductors, hydrophones, stress members and ancillary equipment, is considerable. Therefore, the tubular jacket is filled with a liquid aliphatic hydrocarbon, also known as cable oil, to provide neutral buoyancy for the cable when under tow in the water.
There are problems with seismic streamer cables of conventional type. Because of the liquid content, tube waves are generated that propagate through the cable oil to create undesired noise. Because the streamer cable is under tow in the water, the cable is necessarily under substantial tension and tends to strum like a violin string. The vertical component of such vibration produces troublesome noise, particularly when the hydrophones are employed in pairs in a differential mode as taught by U.S. Pat. No. 4,547,899, which is incorporated herein by reference. An insidious type of noise is caused by so-called bulge-waves. Bulge-waves are due to alternate bulging and contraction of the plastic jacket caused by certain types of externally-applied accelerations and jerk.
There are certain mechanical problems inherent in the hose-like, cylindrical design of conventional streamer cables. Such cables twist and turn about their longitudinal axis when under tow. As taught by the '869 patent, in order to properly resolve the AC and DC components of arriving seismic waves, three or four sensors per unit array were required in order to determine which sensor was "up". Furthermore, to achieve a desired resolution, in the differential mode, the sensors need to be well separated. In a cylindrical cable, the volume of the cable increases as the square of the separation. But physical constraints on the size and capacity of the cable storage reels on the towing ship, severely limit the permissible volume of the streamer cable as a whole, and hence the sensor separation.
Seismic cables for marine use, having a cylindrical configuration, are known, wherein foam or solid floatation material is used for buoyancy. Such cables are confined to bottom-drag or yo-yo operations such as by U.S. Pat. No. 4,314,363. That type of cable is not a true streamer cable and is used only in shallow water.
A flat belt-like cable is disclosed in U.S. Pat. No. 3,921,755. The belt cable is relatively heavy and is confined to use exclusively as a bottom-drag cable. It has no buoyancy means and is designed to lie flat on the sea floor.